<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        div{
            display: flex;
            width:800px;
            height: 400px;
            background-color: pink; 
            flex-direction: row;
            /* 默认的主轴是 x轴 row */
            justify-content: center;
            /* 控制侧轴方向排列方式 */
            /* align-items: flex-start; */
            /* align-items: flex-end; */
            align-items: center;
            /* 了解 */
            /* align-items: stretch; */
        }

        div span{
            width:150px;
            height: 100px;
            background-color: purple;
            color:#fff;
            margin:10px;
        }
    </style>
</head>
<body>
    <div>
        <span>1</span>
        <span>2</span>
        <span>3</span>
    </div>
</body>
</html>